I just overheard a patron say, "Now what is down stage again?"
With that in mind, here are some common theater terms.
Down Stage: Closer to the audience or foot of the stage.
Stage Right: While facing the audience this is the actor's right side, audience's left.
Stage Left: Should be obvious.
Up Stage: The top of the stage furthest from the audience.
Down Stage Center: The middle of the foot of the stage.
Apron: The area of the stage which extends beyond the proscenium.
Proscenium: If the stage is framed, the proscenium is the opening. The curtain hangs in the proscenium.
The House: The area where the audience sits.
Back Stage: The entire area of the theater that is not the stage or the house.
The Wings: The area just off stage where the actors wait to come on stage. The wings are usually partitioned by black curtains that hang from the ceiling to the floor.
Sight Lines: While in the wings, the actor must be aware of sight lines. This means if the actor can see a seat in the house from the wings, the patron in that seat can see them. This can be distracting to the patron and draw focus from the stage to the wings.
Cross: A direction for the actor to move. For example, Maria crossed down-stage-right. This means Maria was up stage and moved down stage, closer to the audience, and to her right.
Ghost Light: A specific lighting fixture that is always left on to please the ghosts of the theater and keep them company.